I filled form 1040 for my 2008 return as married filed as jointly. With that return, I also filled a W-7 form and received an ITIN number for my wife. Then I realized that I made a mistake in my 2007 return and decided to fill an amended return 1040X. In my 2007 return, I also want to choose married filing as jointly and claim deductions for me and for my wife. My question is can I use my wife's ITIN number that I obtained in 2008 to amend my 2007 return?

Yes, if in 2007 you were eligible to file joint return as residents, you can amend your 2007 tax return to joint return. You must both report your worldwide income for 2007.

So did you say yes to my question for using my wife's ITIN backwards in time?

Yes, you CAN use your wife's ITIN (which was issued in 2008) on amended returns for years 2006 and 2007.

You can also use it for 2005 and earlier, but no refund will be forthcoming because you have exceeded the three-year statue of limitations for claiming tax refunds.

This applies to state tax returns as well.